The UK Government's website is among a number giving error messages on Tuesday morning in what appeared to be a worldwide internet issue.
Websites for several leading media companies - including the Guardian, Financial Times, Independent, and the New York Times - were also down.
Anyone attempting to access the gov.uk site - was given the message "Error 503 Service Unavailable".
The Guardian tweeted: "The Guardian's website and app are currently being affected by a wider internet outage and will be back as soon as possible."
Other websites affected included the online discussion platform Reddit, the Evening Standard and French newspaper Le Monde.
Service monitoring website Down Detector registered a spike in reports of outages of Amazon's cloud computing platform Amazon Web Services.
As of 11am on Tuesday, the site recorded 1,700 reports.
Other websites affected included the online discussion platform Reddit, the Evening Standard and French newspaper Le Monde.
The outage appears to have been sparked by an issue with a content delivery network (CDN), a system used to host websites and their content on the internet and serve it to users.
Fastly, one of the world's major CDNs, has reported a major outage across its global network, which is believed to have caused the outage.
The company offers services such as speeding up loading times for websites, protect them from denial-of-service cyberattacks and helping them deal with bursts of traffic in order to stay online and stable.
The US-firm has confirmed it is "currently investigating potential impact to performance with our CDN services".
